Last month, health officials reported that nine children had become infected with the measles virus after visiting Disneyland. Since then, the number has grown to at least 80 individuals. In these situations parents of fully vaccinated children may be nervous about traveling to areas near outbreaks. Should they be concerned? Let's review the facts behind vaccine protection and its limitations.

For many parents, understanding how vaccines work can be a confusing. If a child is vaccinated against measles, why worry about him or her catching it? The answer to this question can be better understood by examining how this particular shot works.

The measles vaccine doesn't provide 100 percent protection to everyone who gets it. It works for most of us, but not all of us. In the case of the Measles, Mumps, and Rubella (MMR) Vaccine, about 95 to 99 percent of people will be protected after they get both recommended doses. That leaves 5 percent unprotected despite being fully vaccinated. In other words, 5 percent of children who are vaccinated against measles can become ill with the disease itself if exposed to it. Here is a graphic from the National Institutes of Allergy and Infectious Diseases that does a good job explaining why an infectious disease is less likely to spread in a highly vaccinated community.

A crowded international destination like Disneyland can produce the "perfect storm" for catching diseases. California has seen an alarming fall in vaccination rates in the past decade. By using loopholes in state laws, parents skipped vaccines meant to keep us all protected. California's low vaccination rates and Disney's big crowds make disease spread very easily. Adding insult to injury, Disneyland is an international tourist destination, a place where folks from every corner of the world visit. Measles is still an ongoing problem in some of those countries.

The majority of those infected last month at Disney were kids who were old enough to receive the vaccine, but never did. Such decisions not to vaccinate, usually based on unfounded fears, put entire communities of children at risk. Measles is usually one of the first diseases to rear its head in unvaccinated populations because it is so highly contagious. Even worse, people may be contagious for four days before they realize they have the disease. All these factors make it a powerful virus that can spread easily and quickly. Doctors worry about measles because it can make children very ill and can sometimes even be fatal.

When considering your own family, if your children have received both doses of the MMR vaccine, they are about 95 percent protected should they encounter someone sick with measles. That should decrease concerns about traveling to areas with active outbreaks. If they are too young to have received both doses, or if their immune system is compromised by disease or medication, these areas should likely be avoided during active outbreaks.

You may have heard that your child can get a blood test to determine if the vaccine has worked successfully to provide him or her with adequate protection ("titers") . For healthy children, I generally do not recommend this test if they have received both MMR shots. The test requires blood work, a trip to the doctor's office (where you might be exposed to other illnesses) and a payment. Two doses of this vaccine should provide you with sufficient confidence that your child is well protected.

Outbreaks are officially reported by the Centers for Disease Control and Prevention and state health departments. You can follow this current outbreak by checking the California Health Department Site or following the national news.

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